In recent years, cements for dental use have contained hydroxyapatite (hereinafter referred to as "HAp and .alpha.-tricalcium phosphate [.alpha.-Ca.sub.3 (PO.sub.4).alpha.-Ca.sub.3 (PO.sub.4).sub.2 : hereinafter referred to as ".alpha.-TCP"] powder and an aqueous solution of polyacrylic acid as a setting solution. A hardened product is made by mixing, and kneading of the powder with the setting solution. However, polyacrylic acid, which does not react during the hardening, sometimes remains and thus, there exists a problem that a body may suffer damage due to elusion of the acid.
Also, for cements for dental use and root canal sealing materials, there have been known cement and root canal sealers in a series of zinc oxide eugenol in which eugenol is mixed to a setting liquid with a purpose of a pain-killing effect. However, cell toxicity has been reported with eugenol and also, a composite resin, that is a material for recovery of a tooth crown part, is disturbed by polymerization with eugenol. Therefore, a number of problems exist for the materials in the eugenol series for dental use.
There has been marketed a bone cement in which a polymer material such as polymethylmethacrylate (PMMA) and methylmethacrylate (MMA) is used. However, the following three problems have been known for this bone cement in which a polymer material is used. First, a bone tissue in a host, which is subjected to plugging up, does not directly combine with the bone cement and, when the bone cement is plugged up in a living body for a long period, there exists a problem such as loosening due to interposition of a fibril tissue. Second, since the temperature is raised to 90.degree..about.100.degree. C. with heat-generation during hardening, there exists a problem that surrounding cells are killed. Third, there exists a problem that elution of a monomer or an oligomer which has not reacted creates a bad effect on the bone.
On the other hand, there have been proposed a number of hardening materials which are furnished with .alpha.-TCP powder or tetracalcium phosphate powder (Ca.sub.4 (PO.sub.4).sub.2 0: hereinafter referred to as "4CP"), which are substances analogous to HAp that is a main inorganic component of body hard tissue, and also furnished with a setting liquid composed of a solution of a type of organic acid. For example, in Japanese official patent provisional publication of Showa 60-36404, there is described a material furnished with .alpha.-TCP powder and a setting solution of 1 M tannic acid. In Japanese official patent provisional publication of Showa 62-12705, there is described a material furnished with .alpha.-TCP powder and a 30.about.60% (w/w) aqueous solution of citric acid. Also, in Japanese official patent provisional publication of Showa 62-83348, there is described a material furnished with .alpha.-TCP powder and a 45% (w/w) aqueous solution of hydroxysuccinic acid. The .alpha.-TCP and 4CP are of high chemical reactivity and can be converted into HAp under the conditions similar to those in a body or a mouth.
The hardening materials described in the above publications have properties such that they cause almost no damage to a body, have the capability to form a hardened product analogous to body hard tissue, and have the capability to combine with hard tissue. A hardening material furnished with .alpha.-TCP or 4CP as a powder component and with a solution of an organic acid as a setting liquid component is very useful for medical and dental use, so that its practical use is very acceptable.
The hardening materials and their hardened products cause no damage to a body, but if a ratio between calcium phosphate powder and the setting solution (hereinafter simply referred to as "powder: liquid ratio") becomes large, the time for hardening becomes extremely short, so that there exists a problem that practical use is not possible.
Hardening materials are classified, for example, in the undermentioned major two classes (a) and (b).
(a) Sealing agent . . . The applications for use are the ones where a very great force does not operate, and the agent is used for plugging up and blocking up a gap or as a firmly fixing supporter, and the time for hardening of the agent is very long. The agent does not show high values in properties of the hardening materials, especially, in resisting force against crushing and it is preferred if the agent slowly releases a substance which has a pain-killing effect. For example, it is used as a bone sealer or a root canal sealer, etc.
(b) Cement . . . The applications for use are ones where a definite weighing force operates, and the agent is used for plugging up and blocking up a gap or with a purpose of joining of hard body tissues to one another, joining a hard body tissue to other material, or between other materials themselves. When a user is going to mix and knead the agent, the time for hardening is properly short and thus, the hardening proceeds in relatively short time, and after hardening, the agent shows a definitely high value in material properties, especially in resisting force against crushing and it is preferred if it could firmly make a chemical bond with a hard body tissue. The examples are bone cement, cement for dental use, and an adhering agent for dental use.
According to the above considerations, the present invention has an object to provide a hardening material which can be applied as the sealer described in the above class (a) and/or the cement described in class (b), undergoes .hardening at about room temperature or body temperature, has no injurious character for a body, and besides, has properties of formation of a hardened product analogous to a hard body tissue and the joining with a hard body tissue, and also whose time for hardening is freely controlled without lowering the working efficiency for mixing and kneading.